OpenCV is a huge open-source library for computer vision, machine learning, and image processing. OpenCV supports a wide variety of programming languages like Python, C++, Java, etc. It can process images and videos to identify objects, faces, or even the handwriting of a human. When it is integrated with various libraries, such as Numpy which is a highly optimized library for numerical operations, then the number of weapons increases in your Arsenal i.e whatever operations one can do in Numpy can be combined with OpenCV.

This OpenCV tutorial will help you learn the Image-processing from Basics to Advance, like operations on Images, Videos using a huge set of Opencv-programs and projects.

Table Of Content:

• Getting Started
• Working with Images
  • Getting Started
  • Image Processing
  • Feature Detection and Description
  • Drawing Functions
• Working with Videos
  • Getting Started
  • Video Processing
• Applications and Projects

Recent Articles on OpenCV !!

Python was developed by Guido van Rossum in the early 1990s and its latest version is 3.10.4, we can simply call it as Python3. Python 3.0 was released in 2008. and is interpreted language i.e it’s not compiled and the interpreter will check the code line by line. This article can used to learn very basics of Python programming language.

So before moving on further.. let’s do the most popular ‘HelloWorld’ tradition 😛 and hence compare Python’s Syntax with C, C++, Java ( I have taken these 3 because they are most famous and mostly used languages).

Note: Please note that Python for its scope doesn’t depend on the braces ( { } ), instead it uses indentation for its scope.
Now moving on further Lets start our basics of Python . I will be covering the basics in some small sections. Just go through them and trust me you’ll learn the basics of Python very easily.

Introduction and Setup

1. If you are on Windows OS download Python by Clicking here and now install from the setup and in the start menu type IDLE.IDLE, you can think it as an Python’s IDE to run the Python Scripts.

   It will look somehow this :
   

2. If you are on Linux/Unix-like just open the terminal and on 99% linux OS Python comes preinstalled with the OS.Just type ‘python3’ in terminal and you are ready to go.
   It will look like this :
   

   The ” >>> ” represents the python shell and its ready to take python commands and code.

   Variables and Data Structures

   In other programming languages like C, C++, and Java, you will need to declare the type of variables but in Python you don’t need to do that. Just type in the variable and when values will be given to it, then it will automatically know whether the value given would be an int, float, or char or even a String.

   
   

   
   
   

   # Python program to declare variables myNumber = 3 print(myNumber)    myNumber2 = 4.5 print(myNumber2)    myNumber ="helloworld" print(myNumber)

   Output:

   3
   4.5
   helloworld
   

   See, how simple is it, just create a variable and assign it any value you want and then use the print function to print it. Python have 4 types of built in Data Structures namely List, Dictionary, Tuple and Set.

   List is the most basic Data Structure in python. List is a mutable data structure i.e items can be added to list later after the list creation. It’s like you are going to shop at the local market and made a list of some items and later on you can add more and more items to the list.
   append() function is used to add data to the list.

   
   

   
   
   

   # Python program to illustrate a list     # creates a empty list nums = []     # appending data in list nums.append(21) nums.append(40.5) nums.append("String")    print(nums)

   Output:

   [21, 40.5, String]

   Comments:

   # is used for single line comment in Python
   """ this is a comment """ is used for multi line comments

   Input and Output

   In this section, we will learn how to take input from the user and hence manipulate it or simply display it. input() function is used to take input from the user.

   
   

   
   
   

   # Python program to illustrate # getting input from user name = input("Enter your name: ")     # user entered the name 'harssh' print("hello", name)

   Output:

   hello harssh   

   
   

   
   
   

   # Python3 program to get input from user    # accepting integer from the user # the return type of input() function is string , # so we need to convert the input to integer num1 = int(input("Enter num1: ")) num2 = int(input("Enter num2: "))    num3 = num1 * num2 print("Product is: ", num3)

   Output:

   Enter num1: 8 Enter num2: 6 ('Product is: ', 48)
   

   Selection

   Selection in Python is made using the two keywords ‘if’ and ‘elif’ and else (elseif)

   
   

   
   
   

   # Python program to illustrate # selection statement    num1 = 34 if(num1>12):     print("Num1 is good") elif(num1>35):     print("Num2 is not gooooo....") else:     print("Num2 is great")

   Output:

   Num1 is good

   Functions

   You can think of functions like a bunch of code that is intended to do a particular task in the whole Python script. Python used the keyword ‘def’ to define a function.
   Syntax:

   def function-name(arguments):
               #function body

   
   

   
   
   

   # Python program to illustrate # functions def hello():     print("hello")     print("hello again") hello()    # calling function hello()

   Output:

   hello
   hello again
   hello
   hello again
   

   Now as we know any program starts from a ‘main’ function…lets create a main function like in many other programming languages.

   
   

   
   
   

   # Python program to illustrate  # function with main def getInteger():     result = int(input("Enter integer: "))     return result    def Main():     print("Started")        # calling the getInteger function and      # storing its returned value in the output variable     output = getInteger()          print(output)    # now we are required to tell Python  # for 'Main' function existence if __name__=="__main__":     Main()

   Output:

   Started
   Enter integer: 5
   

   Iteration (Looping)

   As the name suggests it calls repeating things again and again. We will use the most popular ‘for’ loop here.

   
   

   
   
   

   # Python program to illustrate # a simple for loop    for step in range(5):         print(step)

   Output:

   0
   1
   2
   3
   4
   

   Modules

   Python has a very rich module library that has several functions to do many tasks. You can read more about Python’s standard library by Clicking here
   ‘import’ keyword is used to import a particular module into your python code. For instance consider the following program.

   
   

   
   
   

   # Python program to illustrate # math module import math    def Main():     num = -85        # fabs is used to get the absolute      # value of a decimal     num = math.fabs(num)      print(num)               if __name__=="__main__":     Main()

   Output:

   85.0

   These are some of the most basics of the Python programming language and I will be covering both the intermediate and advanced level Python topics in my upcoming articles.

   This article is contributed by Harsh Wardhan Chaudhary. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks. Your article will be reviewed first by Geeks for Geeks team before publishing.

   Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

   
   
   My Personal Notes arrow_drop_up

   Save

• Division operator
• print function
• Unicode
• xrange
• Error Handling
• _future_ module

Division operator

If we are porting our code or executing python 3.x code in python 2.x, it can be dangerous if integer division changes go unnoticed (since it doesn’t raise any error). It is preferred to use the floating value (like 7.0/5 or 7/5.0) to get the expected result when porting our code. 
 

print function

This is the most well-known change. In this, the print keyword in Python 2.x is replaced by the print() function in Python 3.x. However, parentheses work in Python 2 if space is added after the print keyword because the interpreter evaluates it as an expression. 
 

As we can see, if we use parentheses in python 2.x then there is no issue but if we don’t use parentheses in python 3.x, we get SyntaxError. 
 

Unicode:

In Python 2, an implicit str type is ASCII. But in Python 3.x implicit str type is Unicode. 
 

Python 2.x also supports Unicode 
 

xrange:

xrange() of Python 2.x doesn’t exist in Python 3.x. In Python 2.x, range returns a list i.e. range(3) returns [0, 1, 2] while xrange returns a xrange object i. e., xrange(3) returns iterator object which works similar to Java iterator and generates number when needed. 
If we need to iterate over the same sequence multiple times, we prefer range() as range provides a static list. xrange() reconstructs the sequence every time. xrange() doesn’t support slices and other list methods. The advantage of xrange() is, it saves memory when the task is to iterate over a large range. 
In Python 3.x, the range function now does what xrange does in Python 2.x, so to keep our code portable, we might want to stick to using a range instead. So Python 3.x’s range function is xrange from Python 2.x. 
 

Error Handling:

There is a small change in error handling in both versions. In python 3.x, ‘as’ keyword is required. 
 

__future__ module:

This is basically not a difference between the two versions, but a useful thing to mention here. The idea of the __future__ module is to help migrate to Python 3.x. 
If we are planning to have Python 3.x support in our 2.x code, we can use _future_ imports in our code. 
For example, in the Python 2.x code below, we use Python 3.x’s integer division behavior using the __future__ module. 
 

Output : 
 

1.4 

-1.4 

Another example where we use brackets in Python 2.x using __future__ module: 
 

Output: 
 

GeeksforGeeks 

Refer to this for more details of the __future__ module. 
 

This article is contributed by Arpit Agarwal. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above. 
 

Python Keywords: Introduction

Keywords in Python are reserved words that can not be used as a variable name, function name, or any other identifier.

List of all keywords in Python

We can also get all the keyword names using the below code.

Example: Python Keywords List 

Output:

The list of keywords is : 

[‘False’, ‘None’, ‘True’, ‘and’, ‘as’, ‘assert’, ‘async’, ‘await’, ‘break’, ‘class’, ‘continue’, ‘def’, ‘del’, ‘elif’, ‘else’, ‘except’, ‘finally’, ‘for’, ‘from’, ‘global’, ‘if’, ‘import’, ‘in’, ‘is’, ‘lambda’, ‘nonlocal’, ‘not’, ‘or’, ‘pass’, ‘raise’, ‘return’, ‘try’, ‘while’, ‘with’, ‘yield’]

Let’s discuss each keyword in detail with the help of good examples.

True, False, None

• True: This keyword is used to represent a boolean true. If a statement is true, “True” is printed.
• False: This keyword is used to represent a boolean false. If a statement is false, “False” is printed. 
• None: This is a special constant used to denote a null value or a void. It’s important to remember, 0, any empty container(e.g empty list) does not compute to None. 
  It is an object of its datatype – NoneType. It is not possible to create multiple None objects and can assign them to variables.

Example: True, False, and None Keyword

True
True
3
1
False
False

and, or, not, in, is

• and: This a logical operator in python. “and” Return the first false value. If not found return last. The truth table for “and” is depicted below. 

3 and 0 returns 0 

3 and 10 returns 10 

10 or 20 or 30 or 10 or 70 returns 10 

The above statements might be a bit confusing to a programmer coming from a language like C where the logical operators always return boolean values(0 or 1). Following lines are straight from the python docs explaining this:

The expression x and y first evaluates x; if x is false, its value is returned; otherwise, y is evaluated and the resulting value is returned.

The expression x or y first evaluates x; if x is true, its value is returned; otherwise, y is evaluated and the resulting value is returned.

Note that neither and nor or restrict the value and type they return to False and True, but rather return the last evaluated argument. This is sometimes useful, e.g., if s is a string that should be replaced by a default value if it is empty, the expression s or ‘foo’ yields the desired value. Because not has to create a new value, it returns a boolean value regardless of the type of its argument (for example, not ‘foo’ produces False rather than ”.)

• or: This a logical operator in python. “or” Return the first True value.if not found return last. The truth table for “or” is depicted below. 
   

3 or 0 returns 3 

3 or 10 returns 3 

0 or 0 or 3 or 10 or 0 returns 3 

• not: This logical operator inverts the truth value. The truth table for “not” is depicted below. 
• in: This keyword is used to check if a container contains a value. This keyword is also used to loop through the container.
• is: This keyword is used to test object identity, i.e to check if both the objects take the same memory location or not. 

Example: and, or, not, is and in keyword

Output: 

True
False
False
s is part of geeksforgeeks
g e e k s f o r g e e k s 
True
False

Iteration Keywords – for, while, break, continue

• for: This keyword is used to control flow and for looping.
• while: Has a similar working like “for”, used to control flow and for looping.
• break: “break” is used to control the flow of the loop. The statement is used to break out of the loop and passes the control to the statement following immediately after loop.
• continue: “continue” is also used to control the flow of code. The keyword skips the current iteration of the loop but does not end the loop.

Example: For, while, break, continue keyword

0 1 2 3 4 5 6 
0 1 2 3 4 5 7 8 9 

Conditional keywords – if, else, elif

• if : It is a control statement for decision making. Truth expression forces control to go in “if” statement block.
• else : It is a control statement for decision making. False expression forces control to go in “else” statement block.
• elif : It is a control statement for decision making. It is short for “else if“

Example: if, else, and elif keyword

i is 20

Note: For more information, refer to out Python if else Tutorial.

def

def keyword is used to declare user defined functions.

Example: def keyword

Inside Function

Return Keywords – Return, Yield

• return : This keyword is used to return from the function.
• yield : This keyword is used like return statement but is used to return a generator.

Example: Return and Yield Keyword

45
0
1
3
6
10
15
21
28
36
45

class

class keyword is used to declare user defined classes.

Example: Class Keyword

mammal
I'm a mammal
I'm a dog

Note: For more information, refer to our Python Classes and Objects Tutorial .

With

with keyword is used to wrap the execution of block of code within methods defined by context manager. This keyword is not used much in day to day programming.

Example: With Keyword

as

as keyword is used to create the alias for the module imported. i.e giving a new name to the imported module. E.g import math as mymath.

Example: as Keyword

120

pass

pass is the null statement in python. Nothing happens when this is encountered. This is used to prevent indentation errors and used as a placeholder.

Example: Pass Keyword

Lambda

Lambda keyword is used to make inline returning functions with no statements allowed internally. 

Example: Lambda Keyword

343

Import, From

• import : This statement is used to include a particular module into current program.
• from : Generally used with import, from is used to import particular functionality from the module imported.

Example: Import, From Keyword

3628800
3628800

Exception Handling Keywords – try, except, raise, finally, and assert

• try : This keyword is used for exception handling, used to catch the errors in the code using the keyword except. Code in “try” block is checked, if there is any type of error, except block is executed.
• except : As explained above, this works together with “try” to catch exceptions.
• finally : No matter what is result of the “try” block, block termed “finally” is always executed.
• raise: We can raise an exception explicitly with the raise keyword
• assert: This function is used for debugging purposes. Usually used to check the correctness of code. If a statement is evaluated to be true, nothing happens, but when it is false, “AssertionError” is raised. One can also print a message with the error, separated by a comma.

Example: try, except, raise, finally, and assert Keywords

Output

Can't divide by zero
This is always executed
The value of a / b is :
AssertionError: Divide by 0 error

Note: For more information refer to our tutorial Exception Handling Tutorial in Python.

del

del is used to delete a reference to an object. Any variable or list value can be deleted using del.

Example: del Keyword

Output

20
GeeksForGeeks
NameError: name 'my_variable1' is not defined

Global, Nonlocal

• global: This keyword is used to define a variable inside the function to be of a global scope.
• non-local : This keyword works similar to the global, but rather than global, this keyword declares a variable to point to variable of outside enclosing function, in case of nested functions.

Example: Global and nonlocal keywords

25
20

Note: For more information, refer to our Global and local variables tutorial in Python.

This article is contributed by Manjeet Singh(S. Nandini). If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

Python Keywords – Introduction 
Keywords in Python | Set 1 

More keywords:
16. try : This keyword is used for exception handling, used to catch the errors in the code using the keyword except. Code in “try” block is checked, if there is any type of error, except block is executed.

17. except : As explained above, this works together with “try” to catch exceptions.

18. raise : Also used for exception handling to explicitly raise exceptions.

19. finally : No matter what is result of the “try” block, block termed “finally” is always executed. Detailed article –Exception Handling in Python

20. for : This keyword is used to control flow and for looping.

21. while : Has a similar working like “for” , used to control flow and for looping.

22. pass : It is the null statement in python. Nothing happens when this is encountered. This is used to prevent indentation errors and used as a placeholder
Detailed Article – for, while, pass

23. import : This statement is used to include a particular module into current program.

24. from : Generally used with import, from is used to import particular functionality from the module imported.

25. as : This keyword is used to create the alias for the module imported. i.e giving a new name to the imported module. E.g import math as mymath.
Detailed Article – import, from and as

26. lambda : This keyword is used to make inline returning functions with no statements allowed internally. Detailed Article – map, filter, lambda

27. return : This keyword is used to return from the function. Detailed article –  Return values in Python.

28. yield : This keyword is used like return statement but is used to return a generator. Detailed Article – yield keyword

29. with : This keyword is used to wrap the execution of block of code within methods defined by context manager.This keyword is not used much in day to day programming.

30. in : This keyword is used to check if a container contains a value. This keyword is also used to loop through the container.

31. is : This keyword is used to test object identity, i.e to check if both the objects take same memory location or not. 

Output: 

s is part of geeksforgeeks
g e e k s f o r g e e k s 
True
False

32. global : This keyword is used to define a variable inside the function to be of a global scope.

33. non-local : This keyword works similar to the global, but rather than global, this keyword declares a variable to point to variable of outside enclosing function, in case of nested functions.

Output: 

10
5
5
Value of a using nonlocal is : 10
Value of a without using nonlocal is : 5

 
This article is contributed by Manjeet Singh(S. Nandini). If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.